Process for treating an aqueous medium containing cyclohexanone oxime and cyclohexanone

ABSTRACT

The invention relates to a process for treating an aqueous medium containing cyclohexanone oxime and cyclohexanone, said process comprising stripping the aqueous medium with steam, wherein said stripping is carried out at a pressure higher than 0.11 MPa. The invention also relates to a process for the preparation of cyclohexanone oxime, which involves stripping at a pressure higher than 0.11 MPa.

CROSS REFERENCE TO RELATED APPLICATION

This application is the National Phase of International ApplicationPCT/NL01/00884 filed Dec. 4, 2001 which designated the U.S., and waspublished in the English language.

The present invention relates to a process for treating an aqueousmedium containing cyclohexanone oxime and cyclohexanone. The inventionalso relates to a process for preparing cyclohexanone oxime.

Cyclohexanone oxime can be produced in a process in which a buffered,aqueous medium containing buffer acids or acidic salts, for examplephosphate buffers, and buffer salts derived from these acids, iscontinuously recycled between a hydroxylammonium synthesis zone in whichnitrate or nitrogen oxide is catalytically reduced with molecularhydrogen to hydroxylammonium, and a cyclohexanone oxime synthesis zonewhere hydroxylammonium reacts with cyclohexanone to form cyclohexanoneoxime. After having been enriched in hydroxylammonium in thehydroxylammonium synthesis zone, the aqueous medium is passed from thehydroxylammonium synthesis zone to the cyclohexanone oxime synthesiszone. The cyclohexanone oxime can then be separated from the aqueousmedium that is recycled to the hydroxylammonium synthesis zone. Beforethe aqueous medium is recycled to the hydroxylammonium synthesis zone,it may be enriched with the required nitrate by addition of nitric acidor by absorption of nitrous gases in the aqueous medium in whichinstance nitric acid is formed in situ.

The net chemical reactions occurring during the process can berepresented by the following equations:

-   1) Preparation of the hydroxylammonium:    2H₃PO₄+NO₃ ⁻+3H₂→NH₃OH⁺+2H₂PO₄ ⁻+2H₂O-   2) Preparation of the oxime:

-   3) Supply of HNO₃ to make up the depletion of the source of nitrate    ions after removal of the oxime formed:    H₂PO₄ ⁻+HNO₃→H₃PO₄+NO₃ ⁻

The catalyst used in the preparation of hydroxylammonium is generallypalladium and/or platinum on a carrier material of carbon or alumina.The activity of the catalyst is adversely affected by the presence oforganic contaminants, such as cyclohexanone and cyclohexanone oxime,present in the recycled stream.

Document U.S. Pat. No. 3,940,442 describes a process wherein an aqueousmedium leaving the cyclohexanone oxime synthesis is subjected to astripping step to remove trace amounts of cyclohexanone oxime before itis recycled to the hydroxylamine synthesis zone in order to preventpoisoning of the catalyst in the hydroxylamine synthesis zone. Thisstripping step described in U.S. Pat. No. 3,940,442 is performed in adistilling column at atmospheric pressure.

It is now found that the efficiency of the concentration reduction ofcyclohexanone and cyclohexanone oxime in the aqueous medium is markedlyimproved by increasing the pressure.

Accordingly, the invention provides a process for treating an aqueousmedium containing cyclohexanone oxime and cyclohexanone, said processcomprising stripping the aqueous medium with steam, characterized inthat said stripping is carried out at a pressure higher than 0.11 MPa.

The invention also provides a process for preparing cyclohexanone oxime,said process comprising:

passing an aqueous medium containing phosphate from a hydroxylammoniumsynthesis zone to a cyclohexanone oxime synthesis zone, from thecyclohexanone oxime synthesis zone to a stripping zone and from thestripping zone back to the hydroxylammonium synthesis zone,

in said hydroxylammonium synthesis zone, preparing hydroxylammonium bycatalytically reducing nitrate or nitrogen oxide with hydrogen;

in said cyclohexanone oxime synthesis zone, preparing cyclohexanoneoxime by reacting hydroxylammonium with cyclohexanone;

in said stripping zone, stripping the aqueous medium with steam;characterized in that said stripping is carried out at a pressure higherthan 0.11 MPa.

According to the invention the aqueous medium obtained after strippingmay have a lower concentration cyclohexanone and cyclohexanone oxime. Itis also possible that the aqueous medium to be stripped has an increasedconcentration cyclohexanone and cyclohexanone oxime, with no or onlylimited increase of the concentration cyclohexanone and cyclohexanoneoxime in the aqueous medium obtained after stripping. This isadvantageous since separation steps prior to stripping may be omitted orcarried out to a lesser extent. Moreover, higher amounts ofhydroxylammonium may be converted in the cyclohexanone oxime synthesiszone with no or only limited increase of concentration cyclohexanone andcyclohexanone oxime in the aqueous medium obtained after stripping, andentering the hydroxylammonium synthesis zone. A further advantage of theprocess according to the invention is that less steam may be used toobtain a desired decrease of the concentration cyclohexanone andcyclohexanone oxime. Another advantage is that increased amounts ofcyclohexanone can be obtained in the vapor stream.

According to the invention the stripping is carried out at a pressurehigher than 0.11 MPa. Preferably, the stripping is carried out at apressure higher than 0.13 MPa, more preferably higher than 0.15 MPa, inparticular higher than 0.20 MPa. By increasing the pressure, theefficiency of the reduction of the cyclohexanone and cyclohexanone oximeconcentration in the aqueous medium is further increased. There is nospecific upper limit for the pressure. The stripping may be carried outat a pressure lower than 1 MPa, usually lower than 0.6 MPa. The pressuremay be adjusted by any suitable method, for example by using a pressurevalve.

Preferably, the stripping is carried out at a temperature higher than106° C., more preferably higher than 110° C., in particular higher than115° C., in particular higher than 125° C. There is no specific upperlimit for the temperature. The stripping may be carried out at atemperature lower than 185° C., usually lower than 160° C.

The stripping may be carried out in any suitable stripping zone whereinsteam is passed through the aqueous medium. The aqueous medium and thesteam may be contacted by any suitable method, preferably by contactingthe aqueous medium and the steam in countercurrent flow. A vapor streamcomprising steam and organic compounds discharges from the strippingzone.

The vapor stream may include organic compounds which were originallypresent in the aqueous medium prior to entering the stripping zone, e.g.cyclohexanone, and/or organic compounds which are formed in thestripping zone by conversion of cyclohexanone oxime into other products,in particular into cyclohexanone. Typically, the organic compounds inthe vapor stream include cyclohexanone.

Preferably, the vapor stream discharging from the stripping zone has apressure higher than 0.11 MPa, more preferably higher than 0.13 MPa,more preferably higher than 0.15 MPa, in particular higher than 0.20MPa. There is no specific upper limit for the pressure of the vaporstream discharging from the stripping zone. The vapor stream dischargingfrom the stripping zone may have a pressure lower than 1 MPa, usuallylower than 0.6 MPa. Said pressures refer to the pressure of the vaporstream at the point where it discharges from the stripping zone.

Preferably, the vapor stream discharging from the stripping zone has atemperature higher than 106° C., more preferably higher than 110° C., inparticular higher than 115° C., in particular higher than 125° C. Thereis no specific upper limit for the temperature of the vapor streamdischarging from the stripping zone. The vapor stream discharging fromthe stripping zone may have a temperature below 185° C., usually below160° C. Said temperatures refer to the temperature of the vapor streamat the point where it discharges from the stripping zone.

Preferably, the superficial gas velocity of said steam passing throughthe stripping zone is between 0.2 and 3 m/s, more preferably between 0.4and 1.5 m/s. As used herein the superficial gas velocity refers to thevolumetric steam flow (in m³/s) divided by the free cross sectional areaof the stripping zone (in m²). The residence time of the aqueous mediumin the stripping zone is preferably between 0.5 and 60 minutes.

The steam may be obtained from any source, for instance by evaporatingwater from the aqueous medium. Evaporating part of the water of theaqueous medium may be performed in the stripping zone. It is alsopossible to evaporate part of the water from the aqueous medium beforethe aqueous medium enters the stripping zone or after the aqueous mediumhas been discharged from the stripping zone. In a preferred embodiment,the process comprises obtaining said steam by evaporation of the waterfrom the aqueous medium in an amount of 20-400 kg water per m³ ofaqueous medium, more preferably in an amount of 50-200 kg water per m³of the aqueous medium.

Any suitable vessel may be used as a stripping zone. Preferably, thestripping zone is a column. Preferably, such column is a plate column ora packed column. The plate column may be any suitable column fitted withplates, for instance sieve trays, bubble caps or valve trays.

In a preferred embodiment the aqueous medium contains phosphate,preferably between 2.0-8.0 mol phosphate per liter of aqueous medium.The phosphate may be present as H₃PO₄, H₂PO₄ ⁻, HPO₄ ²⁻ and/or PO₄ ³⁻.Preferably, the aqueous medium is buffered. Preferably, the aqueousmedium is an acidic aqueous medium. Preferably, the aqueous medium to bestripped has a pH of between 0 and 4, more preferably between 0.5 and 4.In a preferred embodiment, the aqueous medium to be stripped contains2.0-8.0 mol phosphate, 0.5-8.0 mol ammonium (NH₄₊) and 0.1-5.0 molnitrate (NO₃ ⁻) per liter of aqueous medium. As used herein thephosphate content refers to the joint content of H₃PO₄, H₂PO₄ ⁻, HPO₄ ²⁻and PO₄ ³⁻ per liter of medium. As used herein the aqueous medium to bestripped refers to the aqueous medium entering the stripping zone.

Preferably, the joint content of cyclohexanone oxime and cyclohexanonein the aqueous medium to be stripped is less than 0.5 wt. %, morepreferably, less than 0.2 wt. %, in particular less than 0.1 wt. %, morein particular less than 0.05 wt. %, in particular less than 0.02 wt. %.Said weight percentages are given with respect to the weight of theaqueous medium. A lower joint content of cyclohexanone oxime andcyclohexanone has the advantage that the tendency for salt loss via thevapor stream is decreased.

The joint content of cyclohexanone and cyclohexanone oxime in theaqueous medium exiting the stripping zone may be less than 0.02 wt. %.Preferably, the joint content of cyclohexanone oxime and cyclohexanonein the aqueous medium exiting the stripping zone is less than 0.01 wt.%, more preferably less than 0.002 wt. %, in particular less than 0.0005wt. %, more in particular less than 0.0002 wt. %, most preferably lessthan 0.0001 wt. %. Said weight percentages are given with respect to theweight of the aqueous medium exiting the stripping zone.

Advantageously, the process comprises causing heat exchange between thevapor discharge stream and a process liquid. This is an effective wayfor using heat of the vapor stream. Preferably, said process liquid is aprocess liquid in a production process for caprolactam. Examples ofsuitable process liquids include organic product comprisingcyclohexanone oxime in an organic solvent, e.g. toluene, or an ammoniumsulphate solution. Preferably, the process comprises withdrawing saidorganic product from the cyclohexanone oxime synthesis zone. Preferably,said heat exchange comprises feeding said vapor stream to a heatexchanger. In a preferred embodiment said heat exchanger is a reboilerof a distillation column, for instance a distillation column whereincyclohexanone oxime is separated from an organic product comprisingcyclohexanone and organic solvent, or a heat exchanger of acrystallizer, for instance a crystallizer wherein water is evaporatedfrom an ammonium sulphate solution to effect crystallization of ammoniumsulphate crystals.

Cyclohexanone may be separated from the vapor stream, for example byphase separation. Separated cyclohexanone may be recycled to thecyclohexanone oxime synthesis zone.

Generally, the concentration of hydroxylammonium in the aqueous mediumentering the cyclohexanone oxime synthesis zone is higher than 0.8mol/l. Preferably, the concentration hydroxylammonium in the aqueousmedium entering the cyclohexanone oxime synthesis zone is higher than1.0 mol/l, more preferably higher than 1.2 mol/l, more preferably higherthan 1.4 mol/l, in particular higher than 1.6 mol/l. Increasing theconcentration hydroxylammonium in the aqueous medium entering thecyclohexanone oxime synthesis zone is advantagesous, since it may forinstance result in an increased conversion of hydroxylammonium and/orsmaller losses of hydroxylammonium by decomposition. An increasedconcentration hydroxylammonium in the aqueous medium entering thecyclohexanone oxime synthesis zone may for instance be achieved byincreasing the residence time in the hydroxylammonium synthesis zoneand/or by increasing the nitrate concentration in the aqueous mediumentering the hydroxylammonium synthesis zone. There is no specific upperlimit for the concentration hydroxylammonium in the aqueous mediumentering the cyclohexanone oxime synthesis zone. Generally, theconcentration hydroxylammonium in the aqueous medium entering thecyclohexanone oxime synthesis zone is below 2.5 mol/l.

We found that an increase of the concentration hydroxylammonium in theaqueous medium entering the cyclohexanone oxime synthesis zone mayresult in an increase of the concentration of organic contaminants, inparticular cyclohexanone and cyclohexanone oxime, in the aqueous mediumexiting the cyclohexanone oxime synthesis zone. The increased amounts oforganic compounds can advantageously be reduced by the process accordingto the invention.

In the cyclohexanone oxime synthesis zone, hydroxylammonium is reactedwith cyclohexanone to form cyclohexanone oxime, preferably in thepresence of an organic solvent. Any suitable organic solvent may be usedin which cyclohexanone and cyclohexanone oxime may be dissolved.Preferably, the organic solvent is selected from the group consisting ofbenzene, toluene, xylene, methylcyclopentane, cyclohexane and mixturesthereof. Most preferably, the organic solvent is toluene. A suitableprocess is for instance described in GB-A-1,138,750. In a preferredembodiment, the reaction of hydroxylammonium with cyclohexanone iseffected by contacting the aqueous medium and an organic streamcomprising cyclohexanone and the organic solvent in countercurrent flow.The cyclohexanone oxime produced may be discharged from thecyclohexanone oxime synthesis zone by any suitable method, preferably bywithdrawing an organic product from the cyclohexanone oxime synthesiszone, said organic product comprising the cyclohexanone oxime and theorganic solvent. The organic solvent and the cyclohexanone may beintroduced into the cyclohexanone oxime synthesis zone at any suitablepoint, preferably downstream of the point where the organic product iswithdrawn from the cyclohexanone oxime synthesis zone (seen in thedirection of flow of the aqueous medium). Most preferably, the organicsolvent and the cyclohexanone are introduced into the cyclohexanoneoxime synthesis zone downstream of the point where the organic productis discharged from the cyclohexanone oxime synthesis zone, and theorganic solvent is introduced downstream of the point where thecyclohexanone is introduced into the cyclohexanone oxime synthesis zone(seen in the direction of flow of the aqueous medium). This embodimenthas the advantage that extraction of residual amounts of cyclohexanoneand cyclohexanone oxime from the aqueous medium is improved. As usedherein, the zone between the point where the organic product leaves thecyclohexanone oxime synthesis zone and the point where the cyclohexanoneis introduced into the cyclohexanone oxime synthesis zone is alsoreferred to as reaction zone. As used herein the zone between the pointwhere the cyclohexanone is introduced into the cyclohexanone oximesynthesis zone and the point where the organic solvent is introducedinto the cyclohexanone oxime synthesis zone is also referred to asextraction zone. For the reaction zone and extraction zone, use may bemade of known types of counterflow reactors, such as for instance pulsedcolumns filled with packing bodies or rotating disc reactors. It is alsopossible to use a system comprising a number, e.g. 3 to 6, ofseries-connected reactors equipped with stirrers, each of these reactorsalso being provided with a liquid-liquid separator. The cyclohexanoneoxime synthesis zone is preferably operated at a temperature between 40to 150° C. Preferably, the reaction medium entering the cyclohexanoneoxime synthesis zone has a pH of between 1 and 6, more preferablybetween 1.5 and 4.

In the hydroxylammonium synthesis zone hydroxylammonium is formed bycatalytic reduction of nitrate or nitrogen oxide with hydrogen. Thehydroxylammonium synthesis zone may be operated at a temperature rangingfrom 20 to 100° C., preferably 30-90° C., more preferably 40-65° C., andat atmospheric, sub-atmospheric or elevated pressures, preferablybetween 0.1 and 5 MPa, more preferably between 0.3 and 3 MPa, and inparticular between 0.5 and 2 MPa (hydrogen partial pressure).Preferably, the pH in the hydroxylammonium synthesis zone is between 0.5and 6, more preferably between 1 and 4. The catalyst employed in thiszone is generally present in a range of between 1 to 25 wt. %,preferably between 5 to 15 wt. % of a precious metal, relative to totalweight of support plus catalyst. Preferably, the catalyst is a palladiumcontaining catalyst, for instance a palladium or a palladium-platinumcatalyst, present on a support, such as for instance carbon or aluminasupport. Generally, the catalyst is present in the hydroxylammoniumsynthesis zone in an amount of 0.2-5 wt. % relative to the total liquidweight in the hydroxylammonium reactor vessel(s). The hydroxylammoniumsynthesis zone is not limited to a specific reactor. A reactor with amechanical stirrer may be used. Preferably, the reactor is column,preferably a bubble column. An example of a suitable bubble column isdescribed in NL-A-6908934.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of a stripping column.

FIG. 2 is a schematic diagram of an embodiment of a process according tothe present invention.

DESCRIPTION OF AN EMBODIMENT

Referring to FIG. 1, A represents a stripping column. To zone A theaqueous medium containing cyclohexanone and cyclohexanone oxime is fedvia line 1. The stripped aqueous medium containing a reduced amount ofcyclohexanone and cyclohexanone oxime leaves the stripping zone A vialine 2. Steam is generated at the bottom of the stripping column by heatsupply (Q) via a heater. Steam contacts the aqueous medium in countercurrent flow and at the top of the column a vapor stream comprisingsteam and cyclohexanone is discharged from the stripping column throughline 3.

Referring to FIG. 2, A represents the stripping zone as described underFIG. 1. The aqueous medium, discharged from the stripping zone A, isrecycled to a hydroxylammonium synthesis zone B via line 2. The aqueousmedium may be enriched with nitrate ions by addition of nitric acidand/or absorption of nitrous gases through line 4. In zone Bhydroxylammonium is formed by catalytic reduction of nitrate withhydrogen. Hydrogen is fed via line 5 to zone B, containing catalyst andnitrate ions; unreacted hydrogen is discharged, with any other gases,via line 6. After being enriched in hydroxylammonium, in zone B, theaqueous medium is passed to the cyclohexanone oxime synthesis zone vialine 7. The cyclohexanone oxime synthesis zone comprises reaction zone Cand extraction zone D. The cyclohexanone to be converted intocyclohexanone oxime in zone C is fed to zone C in an organic solventthrough line 8. The cyclohexanone is introduced into the organic solventvia line 9. The largest part of cyclohexanone oxime produced anddissolved in the organic solvent is removed from the system via line 10.The aqueous medium is passed from reaction zone C to extraction zone Dthrough line 11. Upon passing reaction zone C, the hydroxylammoniumcontent of the aqueous medium has been reduced by reaction and containssmall amounts of cyclohexanone and cyclohexanone oxime. The organicsolvent enters extraction zone D through line 12. Within extraction zoneD, residual cyclohexanone oxime dissolved in an organic solvent isremoved from the aqueous medium through line 13. Through line 1, theaqueous medium leaves the extraction zone D to be fed to the strippingzone A, completing the cycle. The process is carried out continuously.

The invention will be elucidated by the following examples without beinglimited thereto.

EXAMPLES AND COMPARATIVE EXPERIMENTS Comparative Experiment A

An aqueous medium is fed to the upper part of a stripping column, acolumn equipped with 21 sieve trays, a feeding point at the top of thecolumn and a reboiler at the bottom. The dimensions of the strippingcolumn are as follows: a height of 0.8 m and a diameter of 0.025 m. Theaqueous medium entering the stripping column (10 liter/hour) comprises:

-   -   16% by weight of H₃PO₄    -   17% by weight of NH₄NO₃    -   7% by weight of NH₄ H₂PO₄    -   0.8% by weight of hydroxylammonium phosphate    -   0.1% by weight of cyclohexanone oxime and cyclohexanone.

The stripping column is operated under atmospheric pressure, i.e. thevapor stream leaving the column had a pressure of 0.1 MPa and atemperature of 106° C. Steam is generated in the lower part of thecolumn by evaporation part of the acidic aqueous medium (1 kg per hour).Steam comprising water and cyclohexanone leaves the stripping column inan amount of (1 kg per hour). The residence time of the acidic aqueousmedium in the column is 5 minutes. The joint content of cyclohexanoneoxime and cyclohexanone in the aqueous medium leaving the column is0.012 wt. %. The amount of cyclohexanone leaving the column with thesteam is 0.9 wt. %. The results are given in Table I, Example A.

Examples I-IV

The same experiment as described in experiment A is performed exceptthat the stripping column is operated under pressures of 0.12, 0.15, 0.5and 1.0 MPa and temperatures of 110, 116, 157 and 185° C., respectively.The joint content of cyclohexanone oxime and cyclohexanone in theaqueous medium leaving the column is 0.006, 0.002, 0.0004 and <0.0001wt. %, respectively. The amount of cyclohexanone leaving the column withthe steam is 0.9, 1.0, 1.0 and 1.0 wt. %, respectively. The results(given in Table I) show that increasing the pressure results in adecreased joint content of cyclohexanone oxime and cyclohexanone in theaqueous medium leaving the column, and an increased amount ofcyclohexanone in the steam.

Reference Experiment B

The same experiment as described in comparative experiment A isperformed except that the joint content of cyclohexanone andcyclohexanone oxime in the aqueous medium entering the stripping columnis 0.025% by weight. The joint content of cyclohexanone oxime andcyclohexanone in the aqueous medium leaving the column is 0.003 wt. %.The amount of cyclohexanone leaving the column with the steam is 0.2 wt.%. The results are given in Table II.

Examples V-X

The same experiment as described in experiment B is performed exceptthat the stripping column is operated under pressures of 0.12, 0.16,0.17, 0.21, 0.27 and 0.5 MPa and temperatures of 110, 118, 121, 127, 135and 157° C., respectively. The joint content of cyclohexanone oxime andcyclohexanone in the aqueous medium leaving the column is 0.001, 0.0005,0.0003, 0.0002, <0.0001 and <0.0001 wt. %, respectively. The results(given in Table II) show that further increased pressures result in afurther decreased joint content of cyclohexanone oxime and cyclohexanonein the aqueous medium leaving the column.

Experiments 1-3

In these experiments the embodiment as illustrated in FIG. 2 was used toshow the effect of an increasing concentration hydroxylammonium in theaqueous medium entering the cyclohexanone oxime synthesis zone.

Experiment 1

In hydroxylammonium synthesis zone B (containing a catalyst (8 wt. % Pdand 2 wt. % Pt supported on carbon), operated at a temperature of 55° C.at a pressure of 1 MPa (hydrogen partial pressure)) an aqueous mediumhaving the following composition:

-   1.30 mol NH₃OH.H₂PO₄-   1.38 mol NH₄H₂PO₄-   0.665 mol H₃PO₄-   1.73 mol NH₄NO₃-   39.9 mol H₂    (corresponding to a concentration hydroxylammonium in the aqueous    medium of 1.3 mol/l, and a phosphate concentration of 3.34 mol/l)    was produced per unit of time, and continuously fed to reaction zone    C (a pulsed packed column, operated at 55° C.), together with    cyclohexanone and toluene. The molar ratio of hydroxylammonium fed    to the cyclohexanone oxime synthesis zone per unit of time to    cyclohexanone fed to the cyclohexanone oxime synthesis zone per unit    of time, i.e. the ratio hydroxylammonium (in mol/s)/cyclohexanone    (in mol/s) was 0.95. Substantially all hydroxyl ammonium was reacted    to form cyclohexanone oxime. Cyclohexanone oxime dissolved in    toluene was withdrawn from zone C, the cyclohexanone concentration    being 38 wt. % (relative to the sum weight of toluene+cyclohexanone    oxime). The aqueous medium exiting zone C was fed to extraction zone    D (a pulsed packed column, operated at 70° C.), together with    toluene.

The joint content of cyclohexanone and cyclohexanone oxime in theaqueous medium exiting extraction zone D was 0.0043 wt. % (43 ppm).

Experiment 2

In this example all conditions were the same as in the previousexamples, except that the aqueous medium exiting hydroxylammoniumsynthesis zone B and entering reaction zone C had the followingcomposition

-   1.50 mol NH₃OH.H₂PO₄-   1.45 mol NH₄H₂PO₄-   0.39 mol H₃PO₄-   1.65 mol NH₄NO₃-   39.8 mol H₂O    (corresponding to a concentration hydroxylammonium in the aqueous    medium of 1.5 mol/l, and a phosphate concentration of 3.34 mol/l).    The joint content of cyclohexanone and cyclohexanone oxime in the    aqueous medium exiting extraction zone D was 0.0218 wt. % (218 ppm).

Experiment 3

In this example all conditions were the same as in the previousexamples, except that the aqueous medium exiting hydroxylammoniumsynthesis zone B and entering reaction zone C had the followingcomposition

-   1.60 mol NH₃OH.H₂PO₄-   1.45 mol NH₄H₂PO₄-   0.30 mol H₃PO₄-   1.65 mol NH₄NO₃-   39.6 mol H₂O    (corresponding to a concentration hydroxylammonium in the aqueous    medium of 1.6 mol/l, and a phosphate concentration of 3.34 mol/l).    The joint content of cyclohexanone and cyclohexanone oxime in the    aqueous medium exiting extraction zone D was 0.0277 wt. % (277 ppm).

Table III gives an overview of experiments 1 to 3. These experimentsshow that an increase of the concentration hydroxylammonium in theaqueous medium entering the cyclohexanone oxime synthesis zone resultsin an increase of the concentration cyclohexanone and cyclohexanoneoxime in the aqueous medium exiting the cyclohexanone oxime synthesiszone. The increased concentration of cyclohexanone and cyclohexanoneoxime can advantageously be separated from the aqueous medium by theimproved stripping process according to the invention.

TABLE I Result of stripping of an aqueous medium comprising 1000 ppm ofcyclohexanone oxime and cyclohexanone Joint content of concentrationpressure cyclohexanone oxime cyclohexanone of vapor tempera- andcyclohexanone in the steam Exam- stream ture in the aqueous medium afterstripping ple (Mpa) in ° C. after stripping (in wt. %) in wt. % A 0.10106 0.012 0.9 I 0.12 110 0.006 0.9 II 0.15 116 0.002 1.0 III 0.5 1570.0004 1.0 IV 1.0 185 <0.0001 1.0

TABLE II Result of stripping of an aqueous medium comprising 250 ppm ofcyclohexanone oxime and cyclohexanone Joint content of concentrationpressure cyclohexanone oxime cyclohexanone of vapor tempera- andcyclohexanone in the steam Exam- stream ture in the aqueous medium afterstripping ple (Mpa) in ° C. after stripping (in wt. %) in wt. % B 0.10106 0.003 0.2 V 0.12 110 0.001 0.25 VI 0.16 118 0.0005 0.25 VII 0.17 1210.0003 0.25 VIII 0.21 127 0.0002 0.25 IX 0.27 135 <0.0001 0.25 X 0.5 157<0.0001 0.25

TABLE III overview of results of experiments 1 to 3 Concentration Jointcontent cyclohexanone and hydroxylammonium in aqueous cyclohexanoneoxime in aqueous medium entering cyclohexanone medium exitingcyclohexanone oxime synthesis zone oxime synthesis zone Exp. (mol/l)(ppm) 1 1.3 43 2 1.5 218 3 1.6 277

1. A process for preparing cyclohexanone oxime comprising: passing anaqueous medium containing phosphate from a hydroxylammonium synthesiszone to a cyclohexanone oxime synthesis zone, from the cyclohexanoneoxime synthesis zone to a stripping zone and from the stripping zoneback to the hydroxylammonium synthesis zone, in said hydroxylammoniumsynthesis zone, preparing hydroxylammonium by catalytically reducingnitrate or nitrogen oxide with hydrogen; in said cyclohexanone oximesynthesis zone, preparing cyclohexanone oxime by reactinghydroxylammonium with cyclohexanone; in said stripping zone, strippingthe aqueous medium with steam; wherein said stripping is carried out ata pressure higher than 0.11 MPa.
 2. The process according to claim 1,wherein the concentration of hydroxylammonium in the aqueous mediumentering the cyclohexanone oxime synthesis zone is higher than 1.0mol/l.
 3. The process according to claim 2, wherein the concentration ofhydroxylammonium in the aqueous medium entering the cyclohexanone oximesynthesis zone is higher than 1.4 mol/l.
 4. The process according toclaim 1, wherein the stripping is carried out at a pressure higher than0.15 MPa.
 5. The process according to claim 1, wherein the superficialgas velocity of said steam is between 0.2 and 3 m/s.
 6. The processaccording to claim 1, wherein the process comprises obtaining said steamby evaporating water from the aqueous medium.
 7. The process accordingto claim 1, wherein said stripping is carried out in a column.
 8. Theprocess according to claim 1, wherein said column is a plate column or apacked column.
 9. The process according to claim 1, wherein the aqueousmedium is an acidic aqueous medium.
 10. The process according to claim1, wherein the aqueous medium to be stripped has a pH of between 0 and4.
 11. The process according to claim 1, wherein the aqueous medium tobe stripped contains 2.0-8.0 mol/l of phosphate, 0.5-8.0 mol/l ofammonium and 0.1-5.0 mol/l of nitrate.
 12. The process according toclaim 1, wherein the joint content of cyclohexanone oxime andcyclohexanone in the aqueous medium to be stripped is less than 0.5 wt.%.
 13. The process according to claim 1, wherein said stripping iscarried out in a stripping zone, and wherein the process comprisesdischarging a vapor stream from said stripping zone, said vapor streamhaving a pressure higher than 0.11 MPa.
 14. The process according toclaim 13, wherein said vapor stream comprises steam and cyclohexanone.15. The process according to claim 13, wherein the process comprisesexchanging heat of the vapor stream to a process liquid.
 16. The processaccording to claim 15, wherein said process liquid is a process liquidin a production process for caprolactam.
 17. The process according toclaim 16, wherein said process liquid is mixture comprising toluene andcyclohexanone oxime.